Observing Conditions Example: NIRI Imaging of an Extended Object

This is an example of aspects to consider when choosing observing
condition constraints. The science drivers for each program are
distinct, of course, and thus the appropriate set of conditions for
your own observations may be quite different. The example is taken
from the NIRI System Verification plan.

The observation proposed would allow distance determination to a
distant galaxy using the surface brightness fluctuation (SBF)
technique. The SBF procedure requires deep, high spatial-resolution
images of early-type galaxies. The SBF signal increases linearly with
resolution (image quality). Photometric weather is required to put the
measured fluctuation magnitudes on an absolute distance scale. Thus,
the observing conditions I have requested are as follows:

Image quality

- I have requested 70% seeing, expecting
to get good images at K with tip-tilt correction. There is a
trade-off, of course, between integration time required to achieve
a particular S/N and the seeing. I would have requested the best
20% seeing had the galaxy been more distant. However, this image
quality would have been overkill for nearer galaxies and the
likelihood of getting the best conditions is lower. I opted for
the balanced approach of 70% seeing; in a real proposal I would
request 20% conditions for the most-distant subset of my
sample.

Sky transparency (cloud cover)

- Photometric weather is
required to make sure a distance can be computed from an apparent
fluctuation magnitude. The 50% or better sky transparency will
ensure that I get the photometric conditions I require.

Sky transparency (water vapour content)

- in the
near-IR K-band my observations are insensitive to atmospheric
water vapor, so I selected "any" conditions.

Sky background

- for broad K-band imaging, I am not
very concerned about the sky background (OH airglow), and choose
the 80% conditions. Of course, I will be making frequent sky
exposures and subtracting them from my science exposures, and as
long as I do this on time scales of a few minutes, I can remove
the OH airglow and thermal components of the sky background
adequately.

Note
that the statistical likelihood of execution of this observation, if
all of the observing conditions are truly uncorrelated, is 70% * 50% *
100% * 80% = 28% of the time when the target is accessible in the
sky. (In fact we expect some mild correlation and so this is a slight
underestimate). The chances of conditions being favorable for my
observations at any particular time are thus quite small even with my
relaxed image quality constraint. Therefore if I had planned these
observations to be carried out in classically-scheduled time,
statistically I would have required an allocation of four nights to be
'assured' of one night with the conditions I require. Note that 28% is
not the probability of my observations being completed
successfully in the queue since a project will be allocated time when
conditions are right for that project.